The present invention relates to a variable stage type carburetor and, more particularly, to a carburetor of the type provided with a vane controller for controlling the venturi pressure in the heavy load low speed operating phase of engine.
Variable stage carburetors have been publicly known. The carburetor of this type is provided in its intake passage with a throttle valve and a vane interlocked with the throttle valve and constituting a variable venturi portion. Further, means are provided in a fuel passage which opens in the variable venturi portion for varying the fuel metering area in response to the operation of the vane.
The variable stage type carburetor has an advantage, owing to the provision of the vane interlocked with the throttle valve and constituting the variable venturi portion, that a large vacuum can be obtained at the venturi portion, resulting in a good atomization of the fuel.
However, on the other hand, the variable stage carburetor has suffered from a problem that the vacuum established at the venturi portion inconveniently gets low, i.e. gets closer to the atmospheric pressure, during heavy load low speed operation of engine in which the throttle valve is fully opened, due to the interlocking of the vane with the throttle valve, so as to make the rate of the fuel supply undesirably small. This results in a reduced output power of the engine and, in the worst case, stalling of the engine.
In order to avoid this problem, the specification of U.S. Pat. No. 4,005,161 suggests to provide a vane controller which is adapted to preserve the opening degree of the throttle valve, while displacing the vane slightly to smaller opening degree, when the vacuum at the venturi is lowered in the full-throttle-opening operation.
The aforementioned problem in the heavy load low speed operation of engine is effectively overcome by the provision of the vane controller. However, since this vane controller functions to displace the vane to smaller opening degree while preserving the opening degree of the throttle valve, when the throttle valve is opened to the largest opening degree or therearound, the mechanical connection has to include a resilient means such as a torsion spring, for allowing the relative movement between the vane and the throttle valve. Due to the provision of the resilient means, the vane controller requires an unacceptably large actuating power.